In recent years, operating a rotary electric machine in variable speeds using an inverter is carried out actively from the view point of energy saving. However, it has been reported that, when a rotary electric machine is driven by an inverter, various problems arise at insulation parts of the rotary electric machine (Non-Patent Document 1). For example, it has been reported that, if a steep voltage (inverter surge voltage) generated when a switching element inside an inverter turns ON/OFF propagates along a cable and reaches a terminal of the rotary electric machine, then mismatching takes place in surge impedance between the cable and the rotary electric machine, as a result of which the voltage across the terminal of the rotary electric machine jumps to a magnitude of twice an output voltage of the inverter.
Further, it has been reported that, if a steep inverter surge voltage arrives at the inside of the rotary electric machine, then a high voltage is shared at a coil on the end leading side of a rotary electric machine winding or between winding turns in the coil. For the inverter-driven rotary electric machine, therefore, it is necessary to make the insulation design so as to withstand such inverter surge voltages and inspect the fabricated rotary electric machine to find whether or not it has a predetermined dielectric strength.
Incidentally, organic insulating materials are generally used in a low-voltage rotary electric machine having a voltage of 700 Vrms or less. Since such organic insulating materials are poor in resistance to partial discharge (PD), there is the possibility that, if the rotary electric machine is used in a condition under which partial discharge will occur, dielectric breakdown may occur in a comparatively short period of time. Therefore, such an insulation design as permits partial discharge not to occur during operation is conventionally adopted for low-voltage rotary electric machines having a voltage of 700 Vrms or less.
Specifically, the rotary electric machine is insulated in such designs that insulation parts between winding turns, between phases of the rotary electric machine and between the rotary electric machine and the ground have an increased insulation thickness so that partial discharge inception voltages (PDIV) across the insulation parts are higher than voltages applied to the insulation parts of the rotary electric machine upon operation to thereby prevent such partial discharge. Meanwhile, upon inspection of the rotary electric machine fabricated in this manner, a sine wave voltage or an impulse voltage is applied to the rotary electric machine to ensure that no partial discharge occurs at any of the insulation parts between the winding turns, between the phases and between the rotary electric machine and the ground. For example, Non-Patent Document 2 discloses such an insulation design and inspection method as just described. Further, Patent Document 1 discloses a partial discharge measurement method to be used in this instance, for example.